背壓對AZ80鎂合金等徑轉角擠製顯微結構之研究

Abstract

目前有許多的研究使用大量塑性變形法來進行鎂合金機械性質的改善。等徑轉角擠製為眾多大量塑性變形法中最為簡單方便的一種方法，而且運用此法在擠製後可以得到與試片在進行擠製前相同的橫截面。此外，ECAE所需的外力較小，並且可以藉由改變模具參數控制試片的顯微組織。文獻中指出，當溫度越低時晶粒細化的效果越好，但對於不同的材料與擠製模具，最低擠製溫度多半有所限制。而增加背壓有助於增加試片的應變量並且可以防止由張應力造成表面破裂，即代表增加背壓可降低成形溫度。 本研究將探討藉由增加背壓降低最低擠製溫度對鎂合金AZ80晶粒尺寸的影響。實驗結果顯示增加背壓確實有助於降低最低擠製溫度，而在同溫度下增加背壓將使的晶粒尺寸增大，但隨著溫度的降低，晶粒細化的效果越好。

Severe Plastic Deformation (SPD) is widely used to improve the mechanical properties of magnesium. The equal channel angular extrusion (ECAE) is the most simple and convenient technique in SPD. By using ECAE, we obtain a product which has the same cross section with original billet after extrusion .In addition, the force required in extrusion during ECAE process is usually not large, and we can control the microstructure of the billet by changing the die parameters. Our experiments show that low temperature extrusion can achieve in grain refinement, but the lowest extrusion temperature is limited by materials and dies. Imposing a back-pressure is useful for increasing the strain of billet to avoid the cracking on the billet surface by tensile stress. In order words, imposing a back-pressure is beneficial to compromise the extrusion temperature. In this study, the effect on the microstructure of magnesium alloy AZ80 of imposing the back-pressure was investigated. As a result, we found that imposing the back-pressure is helpful in extending the lowest extrusion temperature. At the same temperature, it makes the grain size larger with increasing back-pressure.